A calorie is a unit of heat.
The calorie was originally defined as the amount of heat required at a pressure of 1 standard atmosphere to raise the temperature of 1 gram of water 1° Celsius. The small calorie or gram calorie (usually denoted cal) is the amount of heat energy needed to raise the temperature of one gram of water by one degree Celsius (or one kelvin).
- One kcal is approximately 4.18 kJ (this varies slightly with temperature).
- The specific heat of a substance is the number of calories needed to raise the temperature of one gram by 1oC.